Cooling system for the dry extraction of heavy ashes from boilers

ABSTRACT

The present invention relates to an additional cooling system ( 1 ) for the dry extraction of large flow of heavy ashes produced by boilers ( 100 ) with solid fuel apt to decrease the temperature of the ashes. The system comprises an extractor with metallic belt ( 2 ) gathering the ash which deposits onto the bottom of the boiler ( 100 ), a crushing system ( 3 ), having the purpose of increasing the thermal exchange surface of the material, one or more metallic conveyors ( 4, 6 ) having the cooling function by introducing countercurrent air-flow running through transported ashes, an in-line cooling device ( 5 ) having the function of putting into contact the ash several times with additional countercurrent air in order to increase the possible exchange without necessarily increasing the air-flow entering the combustion chamber. Such additional air can be sent preferably upstream of the air heater or in atmosphere upon fines&#39; captation.

FIELD OF THE INVENTION

The present invention relates to a plant and to a method for the dryextraction and cooling of combustion residues coming from a combustionchamber, in particular for large quantities of heavy ashes originating,for example, from fossil fuel used in thermo-electric energy-producingplants.

BACKGROUND OF THE INVENTION

The constant growth in the request for solid fossil fuels for producingelectric energy makes more frequent the combustion also of coals andlignites with high ash content. The combustion of the latter inhigh-power boilers involves a considerable production of heavy ashesgathered at the bottom of the boiler itself, production which can reachquantities even near to 100 tons/hour. The dry cooling of suchquantities requires large quantities of cooling air, even twice or threetimes greater than the conventional fossil fuels.

As illustrated in EP 0 471 055 B1, in some known ash extraction and drycooling systems, the cooling air, once heated due to the thermalexchange with the latter, is introduced into the boiler from the bottomthereof. Therefore, at first the greater is the quantity of the producedash, the greater is the potential heat recovery which is provided in theboiler by the cooling air in the above-mentioned way.

However, in order to avoid that the combustion efficiency and/or theboiler efficiency be influenced negatively by the air introduced intothe combustion chamber from the bottom rather than from the burners orfrom other specific air entrances and/or in order to avoid similarunwished effects on the production of nitrogen oxides (NO_(x)), someboiler designers prefer limiting this quantity to a maximum value of1.0-1.5% of the total air introduced in the combustion chamber.

For what just illustrated, the known cooling systems do not succeed inimplementing, in an effective and efficient way, the dry cooling ormainly dry, cooling of the heavy ashes and the disposal of the latterand of the related cooling air, above all if such ashes are in largequantities and at high temperature. In particular, even when suchcooling and disposal are succeeded to be obtained, they are achievedwith considerable plant complications and with consequent very highimplementation and handling costs.

Therefore, the technical problem underlying and solved by the presentinvention is to provide a system and method for the dry extraction andcooling of combustion residues coming from a solid fuel combustionchamber which allow obviating to the drawbacks just mentioned with toreference to the known art.

SUMMARY OF THE INVENTION

The above-mentioned problem is solved by a system according to claim 1and by a method according to claim 28.

Preferred features of the present invention are present in the claimsdepending from the same.

The present invention provides some important advantages which will beappreciated in full in the light of the detailed description reportedhereinafter. The main advantage consists in that the invention allows,in case of coals with high ash content, to carry out an adequate andeffective dry cooling of the ash itself without exceeding theabove-mentioned limit of 1.0-1.5% of cooling air introduced in thecombustion chamber from the bottom. Such advantage is particularlyimportant in the above-mentioned case of coals with high content ofheavy ashes. This is obtained mainly by providing an air/ash dryexchanger of gravitational type and allowing only to a controlledquantity of cooling air to be introduced into the combustion chamberfrom the bottom; whereas the air excess coming from such gravitationalexchanger can be discharged into the atmosphere upon dedicatedfiltration, carried to the system for filtering the boiler fumesor—preferably—sent upstream of the combustion air heater, on the fumeside, thus recovering great part of the energy transferred by the ash tothe air.

In order to guarantee the effectiveness in the heat recovery under allconditions of quantity and temperature of the ashes, the cooling airquantity introduced into the system can be adjusted based upon acombination of measures of ash quantity and/or temperature.

Upon summarizing the detailed description of preferred embodimentsreported hereinafter, the present invention mainly relates to anadditional cooling system for the dry extraction of heavy ashes producedby solid fuel boilers, apt to reduce the ash temperature. The systemmainly comprises, arranged subsequentially:

-   -   a metallic belt extractor gathering the ash depositing onto the        boiler bottom; of the type subject of already mentioned patent        EP 0 471 055 B1 and known with the tradename “MAC”;    -   a crushing system, having the purpose of increasing the ashes'        thermal exchange surface;    -   one or more metallic conveyors in line with said extractor,        having the transport and cooling function by introducing air in        countercurrent; and    -   an in-line cooling device, having the function of placing in        contact several times the ash with additional air in        countercurrent in order to increase the possible exchange        without necessarily increasing the air quantity coming back into        the combustion chamber—as mentioned above, such additional air        is then preferably sent upstream of the air heater or in        atmosphere upon captation of fines.

BRIEF DESCRIPTION OF THE DRAWINGS

Other advantages, features and application modes of the presentinvention will be evident from the following detailed description insome preferred embodiments, shown by way of example and not forlimitative purposes. The figures of the enclosed drawings will bereferred to, wherein:

FIG. 1 shows a general layout exemplifying a first embodiment orpreferred operation mode of the invention system, providing to send thecooling air coming from a gravitational air/ash exchanger into the fumeduct associated to the combustion chamber, upstream of the air heater;

FIG. 2 shows a general layout exemplifying a second embodiment orpreferred operation mode of the invention system, providing that the aircoming from a gravity air/ash exchanger is moved by an auxiliary fan anddischarged into the atmosphere;

FIG. 3 shows a general layout exemplifying a third embodiment orpreferred operation mode of the invention system providing to send thecooling air coming from a gravity air/ashes exchanger into the fume ductassociated to the combustion chamber, upstream of the dust removalsystem; and

FIGS. 4 a and 4 b refer to the gravity air/ash exchanger of the previousfigures equipped with an air-dosing system, showing a side view and afront view thereof, respectively.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

By firstly referring to FIG. 1, a system for extracting and coolingcombustion residues, of the kind used for example in solid fossil fuelthermo-electric plants and according to a first preferred embodiment ofthe invention, is designated as a whole with 1. As it will be betterappreciated hereinafter in the description, the system 1 is particularlysuitable to handle large flow of heavy ashes, produced, for example bythe combustion of coals or lignites with high content of ashes.

For greater illustration clarity, the different component of the system1 will be described hereinafter by referring to the path followed by thecombustion residues from the extraction thereof from the bottom of thecombustion chamber (or boiler), designated with 100, to the disposalthereof.

Immediately downstream of the combustion chamber 100, or better of atransition hopper 105 thereof, the system 1 provides a first extractionand/or transport unit, in particular a dry extractor 2 mainly made ofsteel with high thermal resistance. Such extractor 2 is of an alreadyknown type and described for example in EP 0 252 967, hereinincorporated by means of this reference. The extractor 2 gathers theheavy ashes which precipitate downwards into the combustion chamber 100through the transition hopper 105 mentioned above.

Extractor 2, at the side walls of its own casing, has a plurality ofentrance holes for the outer cooling air, distributed in a substantiallyregular way along the development of the extractor 2 itself and each onedesignated with 13. Such entrances 13 preferably are equipped with meansfor regulating cooling air-flow, for example one or more gate valves,apt also to wholly interdict one or more selected entrances.

Cooling air is sucked through the entrances 13 inside the extractor 2and in countercurrent with respect to the ashes' transportation becauseof the depression existing in the combustion chamber 100. More indetail, the air enters thanks to the depression existing in thetransition hopper 105, on the bottom thereof there is a depressionadjusted by the control system of the combustion chamber 100 (generallyaround 300-500 Pa under the atmospheric pressure). Such cooling airinvolving the extractor 2 enters then the boiler 100 from the bottomthereof.

Downstream of the extractor 2 the ashes are fed to a breaker or crusher3, which triturates the coarsest fractions thereof so as to increase thethermal exchange surface and thus to improve the potentials of suchexchange and therefore the cooling process.

Downstream of the crusher 3, the ashes are conveyed to a secondextraction and/or transport unit, in particular a steel-beltconveyor-cooler 4.

Onto the conveyor 4 the ash continues to be cooled by means of air incountercurrent resucked from the outside through additional entrances 13arranged onto the side walls of the conveyor 4 itself in a way analogousto what already illustrated for the first extractor 2. In particular,also at such entrances the air is resucked from the mentioned-abovedepression existing in the combustion chamber 100 and also suchentrances can be equipped with means for flow—regulation of the alreadydescribed kind.

Also the cooling air involving such second conveyor 4 enters the boilerfrom the bottom thereof.

A duct for the passage for the cooling air, designated with 42, can beprovided between the conveyor 4 and the extractor 2 for the bypass ofthe crusher 3.

At this point it will be understood that the system 1 is equipped with adry cooling system, implemented among other things by the air entrances13.

Downstream of the second conveyor 4 such cooling system comprises anair/ash dry gravitational thermal exchanger, preferably of theplate-like type 19, designated as a whole with 5 and shown in greaterdetail in the FIGS. 4 a-4 b. The plates 19 preferably are made ofwear-resistant metal.

Immediately upstream of the exchanger 5 the system 1 can provide anadditional crusher 10, which can be selectively driven in case of need.

Immediately downstream of the gravitational exchanger 5 an additionalcooling air entrance 17 is then provided, equipped with means forcooling air-flow regulation as well, of the already described kind.

Downstream of the exchanger 5 the system 1 further provides a thirdextraction and/or transport unit, in particular a third conveyor 6ending into a silo 11 for discharging the ashes for their disposaland/or possible reuse thereof.

At an entrance portion of the silo 11 an additional air entrance 14 isprovided, equipped with means for cooling air-flow regulation as well,of the already described type.

The additional air of the entrances 17 and 14 crosses in countercurrentthe gravitational exchanger 5 and, considering the air introducedthrough the entrance 14, also the third conveyor 6.

In such gravitational exchanger 5, the ash crushed by the crusher 3 andin case if necessary by the crusher 10 mixes intimately with the airintroduced in countercurrent by the entrances 14 and 17 during the fallsfrom plate to plate, by increasing the thermal exchange and thusincreasing the heat quantity transmitted by the ash to the air. The morethe number of the falls and the air/ashes ponderal ratio are and theless the ash granulometry is, the better the thermal exchange, andconsequently the obtainable cooling degree, will result.

The system 1 then comprises means for sensing the temperature and/orvolumetric and/or ponderal flow of the ashes which in the presentexample are arranged at the ending portion or the exhaust of theconveyor 4 and/or on the main extractor 2 or more preferably at theashes' discharge at the conveyor 6.

The system 1 further comprises control means, in communication with saidsensor means and apt to control the cooling system mentioned above aswell as the extraction and/or transport units 2, 4 and B.

The system 1 then comprises feeding means apt to send part of thecooling air—and in particular the additional air introduced through theentrances 17 and 14 and which crosses the exchanger 5—downstream of theashes' cooling process, into the atmosphere or in a fume duct 101associated to the combustion chamber 100.

In particular, said additional air necessary to cool the ashes in thegravity air/ash exchanger 5 and introduced through the additionalentrances 14 and 17 can follow three different paths depending upon thespecific embodiment or considered construction configuration.

In the here considered case by referring to FIG. 1, the cooling airintroduced through the additional entrances 14 and 17 is resucked,downstream of the countercurrent crossing of the exchanger 5, by thedepression of the fume duct 101 upstream of an exchanger 102 associatedto the boiler 100. Such exchanger 102, usually existing in the knownsystems, is used to pre-heat the combustion air. Said cooling air,heated by the ashes, is then sent in such exchanger 102 (fume side) andused for pre-heating the boiler combustion air.

In the present example, said feeding means comprises then a duct 20 toconnecting the entrance of the exchanger 5 with the fume duct 101. Suchduct 20 must be selectively adjusted and however interdicted/enabled bymeans of an automatic valve 15 (or equivalent means) arranged along itsdevelopment thereof.

The duct 20 then connects, or better is apt to connect, the exchanger 5with the economizers' area of the combustion system, under negativepressure too with respect to the one of the exchanger 5 itself.

Preferably, in order to avoid transporting excessive quantities offines, immediately after the gravity air/ash exchanger 5 the air crossesa cyclone dust collector 7 arranged in line onto the duct 20 and apt todischarge said exceeding fine dusts onto the third conveyor 6.

This configuration allows then an effective recovery of the heat ceasedby the ash to the air during the contact time in the gravity air/ashexchanger 5.

In order to guarantee that the ash cooling process be not influenced onthe extractor 2 and on the conveyors 4 and 6 and in order to avoiduncontrolled air entrance from the boiler bottom, before the entrance ofthe gravity air/ash exchanger 5 (that is upstream of the latter withrespect to the ash flow) a valve with double clapet (not illustrated) oran equivalent pressure control means can be installed, for example adifferential pressure transmitter measured upstream of and downstream ofthe entrance to the gravity air/ash exchanger 5 which, upon acting onthe valve 15 of the duct 20, brings the pressure difference back tozero.

The air flow entering the gravitational air/ash exchanger 5, that is inthe present example the one fed into the system through the entrances 14and 17, can be adjusted by the above-mentioned control means based uponthe ash temperature and/or quantity detected by the sensors mentionedabove, also based upon thresholds which can be set selectively by anoperator managing the system 1.

In the second construction configuration shown in FIG. 2, the additionalcooling air entering the gravitational exchanger 5, downstream ofthereof and of the possible dust remover 7, follows a path differentfrom the one described by referring to the first embodiment. In suchcase, in fact, in the ending tract the air instead of being sucked bythe depression existing in the fume line 101 of the thermo-electricplant, is sucked by a dedicated fan 16 or by equivalent means along aduct 200 equipped with adjusting means 150 to analogous to those alreadydescribed and then discharged into the atmosphere after having passedthrough a dedicated filter 9 arranged upstream of the fan 16. In thiscase a fan work is necessary helping the air to cross at first the thirdconveyor belt 6 and then the gravity air/ash exchanger 5.

In the third construction configuration shown in FIG. 3, the additionalcooling air entering the exchanger 5, upon possible passage into thecyclone 7 mentioned above for the fines separation, is brought to asystem for treating the combustion fumes 104 associated to the boiler100, by entering the fume duct 101 downstream of the air exchanger(heater) 102 mentioned above. In such case, the duct of theabove-mentioned feeding means has been designated with 201 and therelated adjusting means with 151.

It will be understood that even if the configurations of the FIGS. 1, 2and 3 have been described separately, they can exist simultaneously in asame system as different operation modes, which can be activateddepending upon the specific needs.

At this point it will be appreciated that the system 1 has a greatoperating versatility and therefore the capability of handling even verylarge flow of ashes and this without the problems associated to theintroduction of an excessive cooling air-flow from the bottom of theboiler 100. As mentioned above, such versatility is obtained by allowingthe controlled introduction of even very large quantities of cooling airand feeding the additional cooling air-flow (in particular the ratioexceeding 1.0-1.5% of the total combustion air) in the fume duct oroutwards since it is not appropriate to introduce said air-flow in theboiler from its bottom.

The invention has also as object a method for extracting and dry coolingcombustion residues as described so far with reference to the system 1.

The present invention has been so far described by referring topreferred embodiments. It is to be meant that other embodimentsbelonging to the same inventive core may exist, all comprised within theprotective scope of to the herebelow reported claims.

1. A system (1) for extracting and dry cooling combustion residues ofthe type apt to be used in association with a combustion chamber, inparticular for significant flows of heavy ashes deriving for examplefrom fossil fuel in an energy-production plant, which extraction andcooling system (1) comprises: means for extracting and transporting (2,4, 6) combustion residues from the combustion chamber (100, 105); asystem (5, 13, 14, 17) for cooling combustion residues, apt to determinea feeding of cooling air at said extraction and transport means (9, 6,13), the overall arrangement being such that part of said cooling air isintroduced into the combustion chamber (100) from the bottom thereof;and means (20, 15; 200, 150; 201, 151) for feeding another part of thecooling air, downstream of the ash cooling process by the air itself, inatmosphere or in a fume duct (101) associated with the combustionchamber (100).
 2. The system (1) according to claim 1, wherein saidcooling system (5, 13, 14, 17) comprises a dedicated air/ash heatexchanger (5), arranged in line with said extraction and transport means(2, 4, 6).
 3. The system (1) according to the preceding claim, whereinsaid feeding means (20, 15; 200, 150; 201, 151) is apt to feed into thefume duct (101) or into the atmosphere the cooling air crossing saidexchanger (5).
 4. The system (1) according to claim 2 or 3, comprisingone or more dedicated air entrances (14, 17) apt to feed into the system(1) the cooling air which crosses said exchanger (5).
 5. The system (1)according to the preceding claim, wherein said dedicated cooling airentrance or entrances (14, 17) are equipped with means for adjusting theflow of entering air.
 6. The system (1) according to claim 4 or 5,wherein said or at least one (17) of said dedicated entrances of coolingair is arranged immediately downstream of said exchanger (5).
 7. Thesystem (1) according to anyone of the claims 4 to 6, wherein said or atleast one (14) of said dedicated cooling air entrances is arranged at anarea for discharging the ashes (11) of the system (1).
 8. The system (1)according to anyone of claims 2 to 7, wherein the whole arrangement issuch that said dedicated exchanger (5) is crossed in countercurrent bythe cooling air.
 9. The system (1) according to anyone of claims 2 to 8,comprising control means apt to adjust the air flow crossing saiddedicated exchanger (5).
 10. The system (1) according to anyone ofclaims 2 to 9, wherein said dedicated exchanger (5) is of agravitational type.
 11. The system (1) according to the preceding claim,wherein said gravitational exchanger (5) is of the type with multiplefalls.
 12. The system (1) according to claim 10 or 11, wherein saidgravitational exchanger (5) is of the type having multiple plates (19).13. The system (1) according to anyone of the preceding claims, whereinsaid feeding means (20, 15) is apt to feed part of the cooling airupstream of an air/fume exchanger (102) of the fume duct (101) apt topre-heat the combustion air.
 14. The system (1) according to anyone ofthe preceding claims, wherein is said feeding means (20, 15) is apt tofeed part of the cooling air in the economizers' area associated withthe combustion chamber (100).
 15. The system (1) according to anyone ofthe preceding claims, comprising dust removing means (7) of the coolingair portion associated with said feeding means (20, 15; 200, 150; 201,151).
 16. The system (1) according to the preceding claim, wherein saiddust removing means (7) is of the cyclone-like type.
 17. The system (1)according to anyone of the preceding claims, comprising means forcrushing the combustion residues (3, 10) arranged in line with saidextraction and transport means (2, 4, 6).
 18. The system (1) accordingto the preceding claim and according to anyone of claims 2 to 12,wherein said crushing means (3, 10) is arranged upstream of saiddedicated exchanger (5).
 19. The system (1) according to claim 17 or 18,wherein said crushing means (3, 10) comprises a pair of crushers,arranged spaced and in line with said extraction and transport means (2,4, 6).
 20. The system (1) according to anyone of the preceding claims,comprising control means apt to adjust the flow of cooling air enteringthe combustion chamber (100) from the bottom and/or conveyed by saidfeeding means (20, 15; 200, 150; 201, 151).
 21. The system (1) accordingto the preceding claim, wherein said control means is apt to carry outsaid adjustment so that the flow of cooling air entering the combustionchamber (100) from the bottom does not exceed a predetermined amount ofthe total combustion air.
 22. The system (1) according to the precedingclaim, wherein said predetermined amount is equal to about 1.0-1.5%. 23.The system (1) according to anyone of claims 20 to 22 or according toclaim 9, wherein said control means carries out said adjustmentdepending upon the temperature and/or quantity of the combustionresidues.
 24. The system (1) according to anyone of the precedingclaims, comprising pressure control means, arranged in line with saidextraction and transport means (2, 4, 6) and downstream of said feedingmeans (20, 15; 200, 150; 201, 151) with respect to the cooling air flow,apt to prevent an uncontrolled entrance of air towards the bottom of thecombustion chamber (100).
 25. The system (1) according to the precedingclaim, wherein said pressure control means comprises one or more membersselected in a group comprising a valve with double clapet and adifferential pressure transmitter.
 26. The system (1) according toanyone of the preceding claims, wherein said extraction and transportmeans (2, 4, 6) comprises a plurality of extraction and/or transportunits arranged in sequence.
 27. The system (1) according to anyone ofthe preceding claims, comprising an extractor/cooler belt (2) applied tothe bottom of the combustion chamber (100) directly or by transitionhopper means (105), one or more crushers (3, 10), one or moreconveyors/coolers (4, 6), an in-line gravity air/ash exchanger (5)having the purpose of cooling the ash with countercurrent air by meansof falls created with specific plates (19) to increase the exchangesurface between ash and air, a cyclone (7) for the duct collection ofthe cooling air current, a pipeline system (20) for connecting betweengravity air/ash exchanger (5) and the fume duct (101), a conveyor belt(6) downstream of the gravity air/ash exchanger for the ash transport toan end gathering silo (11), and eventually a dedicated fan (16) andfilter (9) for entering the air outcoming from the cyclone (7) into theatmosphere.
 28. A method for extracting and dry cooling combustionresidues coming from a combustion chamber, in particular for significantflows of heavy ashes deriving for example from fossil fuel in an energyproduction plant, which method comprises the steps of: (a) extractingthe combustion residues from the combustion chamber (100, 105); (b)cooling such combustion residues along an extraction and transport path(2, 4, 6) by means of feeding cooling air along the latter, introducing,downstream of the cooling process, part of said air into the combustionchamber (100, 105) from the bottom thereof; and (c) feeding another partof the cooling air, downstream of the cooling process of the ashesthereby, in atmosphere or in a fume duct (101) associated with thecombustion chamber (100).
 29. The method according to claim 28, whereinsaid cooling step (b) provides the use of an air/ash dedicated thermalexchanger (5), arranged along said extraction and transport path (2, 4,6).
 30. The method according to the preceding claim, wherein saidfeeding step (c) provides the feeding into the fume duct (101) or inatmosphere of the cooling air crossing said exchanger (5).
 31. Themethod according to claim 29 or 30, providing a regulation of the airflow crossing said dedicated exchanger (5).
 32. The method according toanyone of the claims 28 to 31, providing regulation of the coolingair-flow entering the combustion chamber (100) from the bottom and/orwhich is involved by said feeding step.
 33. The method according to thepreceding claim, wherein said regulation is carried out so that the flowof cooling air entering the combustion chamber (100) from the bottomdoes not exceed a predetermined amount of the total combustion air. 34.The method according to the preceding claim, wherein said predeterminedamount is equal to about 1.0-1.5%.
 35. The method according to anyone ofthe claims 31 to 34, wherein said regulation is carried out dependingupon the temperature and/or flow of the combustion residues.